Impact of predator dormancy on prey-predator dynamics.

The impact of predator dormancy on the population dynamics of phytoplankton-zooplankton in freshwater ecosystems is investigated using a simple model including dormancy, a strategy to avoid extinction. In addition to recently reported chaos-mediated mixed-mode oscillations, as the carrying capacity grows, we find surprisingly wide phases of nonchaos-mediated mixed-mode oscillations to be present well before the onset of chaos in the system. Nonchaos-mediated cascades display spike-adding sequences, while chaos-mediated cascades show spike-doubling. A host of braided periodic phases with exotic shapes is found embedded in a region of control parameters dominated by chaotic oscillations. We describe the organization of these complicated phases and show how they are interconnected and how their complexity unfolds as control parameters change. The novel nonchaos-mediated phases are found to be large and stable, even for low carrying capacity.

Nested arithmetic progressions of oscillatory phases in Olsen's enzyme reaction model.

We report some regular organizations of stability phases discovered among self-sustained oscillations of a biochemical oscillator. The signature of such organizations is a nested arithmetic progression in the number of spikes of consecutive windows of periodic oscillations. In one of them, there is a main progression of windows whose consecutive number of spikes differs by one unit. Such windows are separated by a secondary progression of smaller windows whose number of spikes differs by two units. Another more complex progression involves a fan-like nested alternation of stability phases whose number of spikes seems to grow indefinitely and to accumulate methodically in cycles. Arithmetic progressions exist abundantly in several control parameter planes and can be observed by tuning just one among several possible rate constants governing the enzyme reaction.

Surfactant-based dispersant for multiwall carbon nanotubes to prepare ceramic composites by a sol-gel method.

A dispersant for multiwall carbon nanotubes (MWCNTs) is proposed that fulfils the requirements of creating a uniform dispersion in the matrix and obtaining a good interface between CNTs and the matrix, and is soluble in generic nonpolar solvents. This dispersant is based on a long chain surfactant, called in this work dabcosil stearate, containing a stearate-based 18-carbon alkyl chain as an anion, and a silsesquioxane containing a bridged, positively charged 1,4-diazoniabicyclo[2.2.2]octane group. It provides not only a very good dispersion medium for the MWCNTs, but also a very good interface between MWCNTs and ceramic matrices, such as alumina and zirconia, prepared by the sol-gel method.

Modulation of the ESIPT emission of benzothiazole type dye incorporated in silica-based hybrid materials.

A set of silica-based hybrid material samples was obtained, with different molar fractions of tetraethylorthosilicate and dimethyldimethoxysilane, as inorganic and organic sol-gel precursors, respectively. The hybrid material was used as a matrix to disperse the 2-(4'-amino-2'-hydroxyphenyl)benzothiazole, a fluorophore with large Stokes shift caused by an excited state intramolecular proton transfer phenomena (ESIPT). UV-vis and steady-state fluorescence in the solid state were applied in order to characterize the photophysical behavior of the dye. The set of samples was characterized by infrared spectroscopy, elemental analysis, and N(2) isotherms. The fluorescent pyrene dye was used as a probe for local polarity environment. For the first time, a modulation in the ESIPT fluorescence emission was achieved, being correlated to the methyl organic content of the hybrid matrix. The equilibrium between the conformers in the ground state is affected by changes in the matrix hydrophobicity. The replacement of silanol groups by methyl groups on matrix surface decreases the hydrogen bond capacity with the dye stabilizing the ESIPT conformer, in spite of the normal conformers, which will present a short wavelength emission band.

Anisotropic self-organization of hybrid silica based xerogels containing bridged positively charged 1,4-diazoniabicycle[2.2.2]octane chloride group.

Anisotropic self-organized hybrid silica based xerogels were obtained. The ordered structure was imposed by the double charged 1,4-diazoniabicycle[2.2.2]octane chloride group bonded in a bridged way. This was confirmed by the presence of well defined X-ray diffraction peaks corresponding to an interplanar distance with the same length estimated for the organic bridged groups. The material was characterized by elemental analysis using CHN technique and the chloride ion was analyzed by a potentiometric titration. (13)C and (29)Si CP MAS solid state NMR spectroscopy and thermogravimetric analysis were also performed. The material that can be obtained in the form of powders and transparent monoliths or films, is thermally stable up to 260 degrees C and the samples with high organic content presented birefringence properties.

Carbon nanotube/silica composites obtained by sol-gel and high-pressure techniques.

The successful incorporation of functionalized single-walled carbon nanotubes (f-SWCNTs) into a silica matrix prepared by the sol-gel method is reported herein. SWCNTs produced through catalytic chemical-vapor deposition (CCVD) have been purified and functionalized with sulfuric, nitric and hydrochloric acids to ensure a good dispersion in an aqueous solution. The nanotube composites are prepared using three concentrations of f-SWCNTs (0.025, 0.050 and 0.075 wt%.) in a silica matrix, resulting in translucent monoliths after gelation. Dense, crack-free and hard compacts are obtained by high-pressure processing at 7.7 GPa and room temperature. Compared to the pure silica compact, compacts containing 0.025 and 0.050 wt% f-SWCNTs show an increased toughness of about 54% and 69%, respectively. The influence of f-SWCNTs on some microstructural aspects of the silica matrix has been studied using nitrogen adsorption/desorption isotherms. Raman spectroscopy has been applied to analyze the effect of the silica matrix and high-pressure compaction on the f-SWCNTs incorporated into the silica matrix. These measurements showed that f-SWCNTs remained in the silica matrix under pressure, suggesting an important interaction with the matrix.

Use of 1,3-diaminepropane-3-propyl grafted onto a silica gel as a sorbent for flow-injection spectrophotometric determination of copper (II) in digests of biological materials and natural waters.

The 1,3-diaminepropane-3-propyl-anchored silica gel (DAPPS) was successfully employed as a sorbent in a spectrophotometric flow system for the preconcentration of Cu(2+) in digests of biological materials (maize powder, soybean, citrus leaves, corn stalks) as well as water samples (river, stream, streamlet, springwater and well). The system presented a minicolumn packed with DAPPS, where the sample solution was passed through it for a period of time, and subsequently, an eluent solution, stripped-out the retained analyte which was further determined with DDTC at 460 nm. The better preconcentration conditions utilized were: 120s loading, 60s elution, 30s regeneration of the column, loading flow rate 6.5 ml min(-1), buffer solution for the preconcentration and regeneration of the column-borate buffer pH 8.5, elution flow rate 2.3 ml min(-1), time of elution 60s, eluent composition, 0.4 mol l(-1) HNO(3). Under these conditions, the preconcentration factor obtained was 36, and the detection limit achieved was 8.4 ng ml(-1) in water samples and 0.84 microg g(-1) in biological material. The maximum adsorption capacity of DAPPS to Cu(2+) was 0.49 mmol g(-1) (31.1 mg g(-1)) obtained in a batch system. The recovery of copper in the water samples ranged from 96.9 to 102.4% and in the biological materials ranged from 97.0 to 102.6%.

7-amino-4-azaheptyl grafted onto a silica gel as a sorbent for the on-line preconcentration and determination of iron(III) in water samples.

A new sorbent was synthesized by anchoring 7-amino-4-azaheptyltrimetoxisilane, freshly prepared, to silica gel, producing 7-amino-4-azaheptyl anchored silica gel (AAHSG). This material was characterized by infrared spectroscopy (IR), elemental analysis (CHN), and nitrogen adsorption-desorption isotherms. Isotherms of the adsorption of Fe3+, Fe2+ and Cu2+ on AAHSG were recorded, which indicated that Fe3+ presents a higher affinity by the sorbent. Therefore, AAHSG was successfully employed as a sorbent in a simple flow system for the preconcentration of Fe3+ in natural water samples, such as, river water, lagoonwater, springwater, stream water, well water and two water reference materials (NIST-SRM 1640, NIST-SRM 1643d). The obtained preconcentration factor was 82.2, and the detection limit achieved was 5.9 ng ml(-1). The recovery of spiked water samples ranged from 95.0 - 103.1%.